The use of volume dampers for controlling air streams is well known. For example, a volume damper is disclosed in U.S. Pat. No. 4,186,719, to Dalsin (Device for Supplying Outside Cold Air to a Fireplace). U.S. Pat. No. 4,236,668 to Prikkel discloses a thermally responsive damper assembly having position controls, for mounting in a flue which vents gases from a region in which combustion is to occur. U.S. Pat. No. 4,399,806 to Love discloses a fireplace draft control, or volume damper. U.S. Pat. No. 4,691,689 to Shepherd discloses a one-piece adjustable damper. U.S. Pat. No. 4,669,499 to Miyake et al. discloses a valve casing for use in a butterfly valve, the valve casing having a double structure comprising an inner shell and an outer shell. U.S. Pat. No. 5,207,615 to Edmisten discloses a damper assembly for an air plenum system having a plurality of air duct lines. U.S. Pat. No. 5,257,772 to Habicht discloses a sanitary butterfly valve including a closable disc member carried by a resilient valve seat. The valve seat is compressed between a housing and clamp plate while being retained by a first acute lip and a second acute lip. U.S. Pat. No. 3,750,698 to Walchle et al. discloses a coated valving member for a butterfly-type valve, the valving member being coated with a protective coating material by a high-pressure and high-temperature molding process. The body plate of the coated valving member has a plurality of openings extending through the body plate. The protective coating material fills and extends through the openings to integrally attach the coating on opposite sides of the body plate. Thus both the basic concept of a volume damper and its general uses in various applications are known.
While each of these patents discloses volume dampers which fulfill their respective particular objectives and requirements and are most likely quite functional for their intended purposes, none of the background art discloses an apparatus that allows for the installation of a corrosion-resistant volume damper in an existing duct system without the necessity of custom-making the damper and/or custom-cutting the duct. Neither is there any volume damper especially adapted for easy installation between existing duct flanges. There still exists the need for a new and improved chemical-resistant and corrosion-resistant volume damper apparatus which is easily and inexpensively installed in new or existing exhaust air systems that are designed to handle a wide range of corrosive exhaust streams. In this respect and others, the invention disclosed herein substantially fulfills this need.